Inhibition of thymidylate synthetase and dihydrofolate reductase by naturally occurring oligoglutamate derivatives of folic acid.

Naturally occurring oligoglutamate derivatives of folic acid in extracts of Escherichia coli have been isolated on the basis of their inhibitory actions toward thymidylate synthetase and dihydrofolate reductase. The inhibitor of thymidylate synthetase has been identified as N-5-formyl-H4pteroyloligoglutamate (approximately 5 amino acid residues). It is 150-fold more inhibitory than the monoglutamate. Synthetic N-5-formyl derivatives containing 3 to 6 glutamyl residues were prepared and found to be 67- to 200-fold more inhibitory than the monoglutamate. N-5-Formimino-H4pteroyltriglutamate is one-twentieth as inhibitory as the corresponding N-5-formyl derivative. The inhibitor of mouse leukemia dihydrofolate reductase has been identified as N-10-formylpteropentaglutamate. It is approximately 7 times as inhibitory as N-10-formylpteroylmonoglutamate. It is 4,400 times as inhibitory toward mouse leukemia dihydrofolate reductase compared with the enzyme from E. coli. Lysine analogs of N-5-formyl-H4folate containing alpha0glutamyl groups in peptide linkage to the epsilon-amino group of lysine were relatively poor inhibitors of thymidylate synthetase. The inhibitory action of folic acid oligoglutamates on E. coli thymidylate synthetase was subject to reversal with 0.4 M NaCl, an effect that was more marked with various pteroyloligoglutamates than with H4homopteroylmonoglutamate and N-5, N-8-deaza-N-10-methylpteroylmonoglutamate.